A white-emitting organic EL device is used in lightings, a full-color display provided with a color filter, or the like. A display utilizing an organic EL device that emits while light has been actively developed since the production thereof is easy as compared with a display obtained by fabricating a red-emitting organic EL device, a green-emitting organic EL device and a blue-emitting organic EL device separately.
As the method for producing an organic EL device that emits white light, there is a method in which a plurality of emitting layers that emit different colors of light are stacked, and white color emission is obtained by overlapping of emission of each emitting layer. In this method, by forming an intermediate layer between the emitting layers, blocking of electrons or holes that causes light to be emitted mainly from one emitting layer can be prevented, whereby emission can be obtained from each emitting layer in a well-balanced manner.
For example, in Patent Document 1, an organic EL device is disclosed in which a red-emitting layer, a green-emitting layer and a blue-emitting layer are stacked from the anode side and an intermediate layer comprising α-NPD is formed between the green-emitting layer and the blue-emitting layer. Due to the formation of the intermediate layer, each emitting layer is caused to emit light in a well-balanced manner.
However, there is a problem mentioned below. Since α-NPD is a hole-transporting material, when α-NPD is used in the intermediate layer, while highly efficient emission can be obtained by blocking electrons, due to excessively strong blocking properties of α-NPD, electrons tend to be accumulated in the interface between the emitting layer on the cathode side of the intermediate layer and the intermediate layer, and the emitting layer on the cathode side of the intermediate layer tends to be deteriorated easily, whereby the life of the organic EL device is shortened.
Further, electrons are blocked strongly, electrons cannot be distributed to the emitting layer positioned on the anode side of the intermediate layer as long as the thickness of the intermediate layer is not allowed to be extremely thin (e.g. 3 nm). Therefore, film thickness control is significantly difficult, posing a problem in respect of mass production.
Further, there is another problem mentioned below. Since emission balance of each emitting layer is sensitive to changes in thickness of the intermediate layer, the film thickness margin of the intermediate layer becomes very small. As a result, film thickness control on the order of several angstroms becomes necessary, resulting in poor mass productivity.
On the other hand, if an electron-transporting material is used in an intermediate layer, there is a concern that, since it has a poor hole-transporting capability, poor emission is obtained from an emitting layer positioned on the side nearer to the cathode than the intermediate layer, whereby luminous efficiency may be lowered. In order to obtain well-balanced white emission, it is required to control the thickness of the intermediate layer on the order of several angstroms, resulting in poor mass productivity. When a material having a small energy gap is used in the intermediate layer in order to relax charge-blocking properties, an exciton energy in the adjacent emitting layers cannot be confined, thereby causing lowering in luminous efficiency.